JP 2013-21826A (Reference 1) discloses a technology in which a permanent magnet (a rotor magnet in the reference) is inserted into a magnet arrangement groove that is formed in a rotor yoke (a magnet back yoke in the reference) configuring a rotor, as an electric motor which is configured as described above. In the magnet arrangement groove, an overhang portion is formed to have a top portion width on an outer circumference side narrower than a bottom portion width on a rotary shaft core side. The permanent magnet is formed to have substantially the same cross-sectional shape as a space shape of the magnet arrangement groove.
In Reference 1, a shape of an outer side surface of the permanent magnet is formed to be a cylindrical surface shape which is in common with an outer circumferential surface of the rotor yoke. An outer surface thereof is exposed in a state where the permanent magnet is inserted into the magnet arrangement groove portion, and the overhang portion comes into contact with the permanent magnet, thereby hindering movement thereof when the rotor rotates. The permanent magnet is fixed to the magnet arrangement groove by using an adhesive.
JP 2009-44797A (Reference 2) discloses a technology in which a plurality of the permanent magnets (rotor magnets in the reference) are arranged on the outer circumference of the rotor yoke (a rotor core in the reference) in a circumferential direction and are fixed to the rotor yoke by using an adhesive, and cylindrical protection materials are arranged on the outer circumference of the permanent magnets.
JP 2012-257433A (Reference 3) discloses a technology in which even though the permanent magnet is not configured to be held by the rotor on the outer circumference thereof in an exposed state, the permanent magnets are arranged to be inserted into a plurality of accommodation holes which are formed at a predetermined interval in the circumferential direction in the vicinity of the outer circumference of the rotor yoke (a rotator in the reference) configuring the rotor.
In Reference 3, the outer side surface and a rear side surface of the permanent magnet are formed to be cylindrical surfaces having the rotary shaft core of the rotor as the center. Pole-to-pole step portions to be formed in portions of the rotor are arranged between the adjacent permanent magnets. Thus, a magnetic flux is directed toward a stator side without generating any closed magnetic flux at end portions of the permanent magnets. As a result, demagnetization durability is improved.
As disclosed in Reference 1, in a configuration in which an outer circumferential surface of a permanent magnet held by a rotor on an outer circumference thereof is formed to have a cylindrical surface shape equivalent to an outer circumferential surface of the rotor having a rotary shaft core as the center, the outer circumferential surface of the permanent magnet can approach an inner circumference of a stator. Thus, an action of a strong magnetic field realizes powerful rotations.
As disclosed in Reference 1, in a configuration in which a portion of the permanent magnet is caused to come into contact with a rotor yoke so as to be prevented from falling off, the permanent magnet can be reliably held in an appropriate position, compared to a case of fixing the permanent magnet by using an adhesive as disclosed in Reference 2.
However, as disclosed in Reference 1, when shapes of end surfaces of the permanent magnet are set so as to come into contact with an overhang portion of the rotor yoke, the end surfaces of the permanent magnets which are adjacent to each other in a circumferential direction become nonparallel. This configuration differs from that described in Reference 3. Thus, a magnetic flux extending outward from the end surfaces of the permanent magnet is closed, resulting in deterioration of demagnetization durability.
In the electric motor, filling a gap between the permanent magnet and the rotor yoke with a resin is also performed. However, as disclosed in Reference 3, when an epidermis portion is formed on the outer circumference of the rotor, it is assumed that the epidermis portion is outwardly deformed due to condensation pressure of the resin.
Particularly, in order to configure a powerful electric motor, a strong magnet which is made by including rare-earth elements such as samarium, neodymium, and dysprosium in the permanent magnet is used. However, the permanent magnet including the rare-earth elements is expensive, causing an increase in a manufacturing cost of the electric motor. Therefore, there is a demand of compactification of the permanent magnet to be used from a viewpoint of reducing the cost.